1. Field of the Invention
This invention relates to a method of treating sewage sludge and more particularly to a method of pyrolyzing sewage sludge, which requires less installation and maintenance costs and which can treat the sewage sludge without any risk of involving subsidiary environmental pollution.
2. Description of the Prior Art
An incineration method has heretofore been widely used as the most general method of treating sewage sludge produced from sewage purification plants.
That is, as shown in FIG. 1, a sludge cake 1, which is dehydrated up to 65 to 85% of moisture, is fed into the uppermost stage of a multi-hearth incinerator 2. For example, the amount of heat required for drying and incineration is generated in a combustion chamber 3 and is fed as a high temperature hot gas through a duct 4 into the lower part of the multi-hearth incinerator 2. The sludge 1 is completely burned to ashes at the lower stage of the multi-hearth incinerator 2 and the ashes 5 are extracted downwardly therefrom.
The combustion gas of the sludge rises in the multi-hearth incinerator 2 and is directly brought into contact with the dehydrated sludge to dry it and then exhausted from the uppermost part of the multi-hearth incinerator 2 as exhaust gas.
This exhaust gas, however, emits a strong offensive odor, so that if the exhaust gas is directly discharged into atmospheric air, the strong offensive odor thereof causes subsidiary environmental pollution. In order to eliminate such disadvantages, the exhaust gas is subjected to a deodorizing treatment. For this, purpose, the exhaust gas is fed through a preheater 8 to an after-burner 6 by which the exhaust gas is heated at a temperature of 750.degree. to 800.degree. C and then fed through the preheater 8 to an exhaust gas treating installation 7 which can effect cooling, dust removing, desulfurization etc. and subsequently the exhaust gas is discharged into atmospheric air.
Such a conventional method of treating sewage sludge by the incineration requires a large amount of combustion air for completely burning the sewage sludge to ashes, which is several times larger than the theoretical amount. As a result, it is inevitable that a large amount of exhaust gas is generated. Generation of such a large amount of exhaust gas has a number of disadvantages. In the first place, the exhaust gas treating installation 7 becomes so large in size and complex in construction as to excessively increase the installation cost and the maintenance cost. Secondly, a large amounts of fuel are required to effect after-burning for the deodorizing treatment, thereby increasing both the installation cost and the maintenance cost. Finally, in the case of incinerating a chromium compound containing sewage sludge, hexavalent chromium injurious to the human body is produced and remains in burned ashes whereby the burned ashes must be subjected to after treatment for the purpose of preventing environmental pollution.